1. Field of the Invention
The invention relates to a method for measuring a program-controlled machine tool using a measuring sphere.
2. Background Discussion
To comply with the high accuracy requirements of modern machine tools an accurate measurement of the individual machine components during the production process and of the whole machine after its production is indispensable so that possible defects can be detected in an early stage and compensated for by corresponding corrective measures, if required. After certain periods of operation further measurements are necessary to detect and analyze the state of wear of individual machine components as well as defects caused by improper handling.
For the purposes mentioned above various measuring methods are known with the aid of which, for example, the pivot length, the displacement of the axis of a spindle head or a round turntable, as well as the slack, can be detected by special measurement programs. To carry out the individual measuring operations in milling and drilling machines, one method employs a sensing element mounted in the work spindle which is used in connection with a polished block clamped to the machine part to be measured, particularly the workpiece table. Such measuring methods are extremely time-consuming and require numerous rectification and measuring operations.
For measuring the rotational axes of a milling machine, for example, of milling heads of various types, a method referred to as FIDIA head measuring system is known in which a special calibration and measuring device is used. With this known measuring method a positioning error compensation for the two round axes as well as the automatic detection of their zero point can be carried out. In addition, an examination of the parallelism of the rotational planes of the round axes with respect to the linear axes, as well as an extended volumetric compensation, can be carried out. The calibration and measuring device required to accomplish this method comprises a stator to be mounted on the machine part, for example, the machine table, and includes three struts angularly displaced by 120° and carrying sensing elements positioned in a predetermined angle of inclination at their upper ends. A measuring tool comprising a cylindrical shaft and a measuring sphere disposed at its free end is mounted in the work spindle of the machine. The measuring sphere is positioned by corresponding movements of the work spindle so that all three sensing elements contact the surface of the sphere. This known measuring method is also connected with certain expenditures since relatively complex calibration and measuring devices are required.
German patent 31 32 383 C describes a device and a method for testing the measuring accuracy of coordinate measuring devices. Two measuring spheres machined with a high surface quality are disposed on a common carrier in an adjustable spacing and provided with bores along a measuring beam. Changes of the distance between the spheres are measured by an interferometer arrangement. The exact original distance between the two measuring spheres needs to be known. The spatial position of the two measuring spheres can be detected by the coordinate measuring device by means of a multiple point sampling. A control computer detects and compares the distance values of the measuring spheres detected by the laser interferometer and the values measured by the coordinate measuring device during the sampling of the spheres. Measuring machine tools, particularly the round axes, is not possible.